1. Field of the Invention
The present invention relates to a method of and apparatus for inspecting a printed board, and more particularly, it relates to an improvement in detecting neck disconnection between a land and a line of a wiring pattern.
2. Description of the Prior Art
With reduction in size and weight as well as improvement in performance of electronic components, a wiring pattern of a printed board is also refined and highly densified, leading to the need for reduction in line width and through hole diameter. In particular, a through hole called a mini via hole, whose diameter is reduced to 0.5 to 0.1 mm as compared with a general value of 0.8 mm, is currently used in a multilayer board for attaining electric connection between major surfaces of the board.
With such reduction in through hole diameter, new techniques are desired in various aspects such as plating, drilling and reliability testing for the through hole.
In general, drilling is not as accurate as process photoetching process, and the through hole tends to deviate from the wiring pattern. A through hole of about 0.8 mm in diameter is provided on its periphery with a sufficiently wide land, so that electrical reliability of the printed board is hardly influenced by slight misregistration of the through hole.
As the land is reduced following reduction of the through hole diameter, however, it is difficult to ensure accuracy for reliably providing a through hole in the land. Thus, electrical reliability of the printed board is disadvantageously lowered by misregistration of the hole, and hence importance of inspection for such misregistration of the through hole is increased.
Such inspection for hole misregistration requires electric inspection as well as appearance inspection. While an inspector for detecting leakage light from plating cracks is known in relation to appearance inspection, various problems arise with the development of a multilayer board. Further, this inspector cannot be applied to inspection of pattern disconnection which is caused by relative misregistration between a through hole and a pattern.
FIGS. 1 and 2 illustrate relative positional relations between a land R and a through hole H. Referring to FIG. 1, the center 0 of the through hole H is registered with the center of the land R, to form an excellent pattern P. Referring to FIG. 2, on the other hand, the center O of the through hole H deviates from the center of the land R, such that a part of the through hole H outwardly projects from the land R. The size of this projecting part is obtained by an angular aperture .theta.. When the angular aperture .theta. is larger than a predetermined reference value, the pattern disconnection is determined to be defective. More particularly, a ring image RP (FIG. 3) is obtained from the image representing the through hole, and an area WR on which the ring image RP overlaps an image RI representing the land is specified. The aperture angle .theta. is obtained from the cut portion of the area WR.
As described above, defectiveness of pattern disconnection can be determined by obtaining the angular aperture .theta., and there has been proposed a technique which is aimed at automation of such a decision.
However, when defectiveness is determined by measuring the angular aperture .theta., it is difficult to detect the so-called "neck disconnection", i.e., disconnection of a land R and a line L as shown in FIGS. 4 and 5. For example, a method of measuring an angular aperture .theta., which is disclosed in Japanese Patent Application No. 1-82117 (1989) filed in the name of the applicant, is adapted to obtain an enlarged contour RP by enlarging the contour of a through hole H and observe the state of overlapping of through hole H and a land R, thereby measuring an angular aperture .theta. (=.theta..sub.1 +.theta..sub.2) of a non-overlapping portion. In this case, however, it is impossible to make a decision as to neck disconnection although defectiveness can be decided. Thus, a technique for determining the presence of a neck disconnection on a printed board is not known.